The present disclosure relates to a heat exchanger system for treatment of a flow of exhaust gases in an exhaust gas aftertreatment system of a vehicle. The heat exchanger system comprises a nitrogen monoxide (NO) oxidation site for oxidising nitrogen monoxide to nitrogen dioxide (NO2). The present disclosure also regards an exhaust gas aftertreatment system and a vehicle comprising such a heat exchanger system, and a method for using such a heat exchanger system.
For meeting the legal emission requirements emission control and exhaust gas aftertreatment systems are generally necessary. Usually, such an exhaust gas aftertreatment system comprises an oxidation catalyst for oxidizing nitrogen monoxide (NO) to nitrogen dioxide (NO2), a particulate filter and a unit for reducing nitrogen oxides (NOx) emissions.
These devices are usually arranged as separate units in series, although many versions of combined configuration also appear. Since the different units influence each other, it has been proven as advantageous to arrange the oxidation catalyst upstream of the particulate filter and upstream the NOx reduction unit. This is due to the following reasons:
1. The oxidation reaction is exothermic, which means that heat is produced, which in turn increases the exhaust gas temperature. For reducing the deposition of soot in the particulate filter, it is desired to operate the particulate filter with exhaust gas having a high temperature. Consequently, the additional heat produced by the exothermic oxidation reaction may be used for increasing the efficiency of the particulate filter.
2. When using an Selective Catalytic Reduction (SCR) unit as NOx reduction unit, a NO:NO2 ratio in the vicinity of or approximately 50:50 is preferred for operating the SCR unit with high efficiency. Since exhaust gas exiting the internal combustion engine usually has a higher amount of NO than of NO2, it is advantageous to adapt the NO:NO2 ratio to the desired approx. 50:50 ratio by means of the upstream arranged oxidation catalyst.
Disadvantageously, regardless which arrangement of the different units is chosen, the number of different units arranged in the exhaust flow may result in a bulky and heavy exhaust gas aftertreatment system.
For further increasing the efficiency of the particulate filter and for further exploiting the heat produced by the exothermic reaction, it has also been suggested, e.g. in DE 102 21 174 B4, to include a counter-flow heat exchanger into the exhaust gas aftertreatment system for exchanging heat between exhaust gas streaming to the particulate filter and exhaust gas streaming from the particulate filter. Additionally, it has been suggested to include the oxidation catalyst into the heat exchanger.
Disadvantageously, by including the oxidation catalyst into the heat exchanger, the conversion of NO to NO2 cannot be controlled in a sufficient way. Moreover, the additional heat exchanger increases the number of devices used in the exhaust gas aftertreatment system.
It is desirable to provide a heat exchanger system which provides an improved NO:NO2 ratio control, which may be utilised e.g. in devices located downstream of the heat exchanger system.
According to a first aspect of the present, disclosure a heat exchanger system for treatment of a flow of exhaust gases in an exhaust gas aftertreatment system of a vehicle is disclosed. The heat exchanger system comprises a nitrogen monoxide (NO) oxidation site for oxidising nitrogen monoxide (NO) to nitrogen dioxide (NO2). The NO oxidation site is positioned such that the flow of exhaust gases at a downstream end of the NO oxidation site in use of the heat exchanger system is arranged to proceed at a temperature within as predetermined temperature interval corresponding to a desired NO to NO2 (NO:NO2) ratio interval in the flow of exhaust gases.
When the flow of exhaust gases from an engine, to which the heat exchanger system in use may be coupled, passes over the nitrogen monoxide (NO) oxidation site, nitrogen monoxide in the exhaust gases is oxidised into nitrogen dioxide. The oxidation process is temperature dependent. During relatively seen cold temperatures a majority of the nitrogen monoxide can from a thermodynamically standpoint be oxidised into nitrogen dioxide, where after increasing temperature only a small fraction of the nitrogen monoxide from a thermodynamically standpoint can be oxidised into nitrogen dioxide. On the other hand, at relatively seen cold temperatures the kinetics makes the oxidation of the nitrogen monoxide a slow process even with the best catalysts, and hence only a small fraction is oxidised into nitrogen dioxide. Increasing the temperatures, the kinetics makes the process faster and more nitrogen monoxide can be oxidised until it reaches the thermodynamically limits. Hence the amount of nitrogen monoxide which is oxidised into nitrogen dioxide is related to the temperature like an inverted V, where the peak of oxidation lies around approximately 250-400° C. and the resulting NO:NO2 ratio is between 70:30 and 20:80 (and thus the NO2 NOx ratio is between 30 and 80%), depending on catalyst and gas conditions. Reference here is made to FIG. 3, curve C. The curve describes an equilibrium state at each temperature, such that at each temperature, under otherwise optimum conditions, a known amount of nitrogen monoxide is oxidised into nitrogen dioxide which gives a certain NO:NO2 ratio. If the conditions are less than optimal the curve is still relevant, although the amount of nitrogen monoxide which is oxidised may be slightly less, but still at a known or foreseeable level. Knowing the appearance of the curve and the conditions under which it is operable it is possible according to the present disclosure to locate the NO oxidation site within the heat exchanger system such that a desired NO:NO2 ratio interval is achieved in the flow of exhaust gases when they pass over the NO oxidation site, and more particularly, when they proceed therefrom. The present disclosure hence gives the advantage of control of the nitrogen monoxide oxidation through placing in particular the downstream end of the nitrogen monoxide site at a location where a temperature interval of the heat exchanger system corresponds to the desired NO:NO2 ratio interval. The equilibrium curve thus indicates a temperature interval which is appropriate for the location within the heat exchanger system. When knowing the temperature distribution within the heat exchanger system it is consequently possible to reliably locate the NO oxidation site there within.
According to an embodiment the heat exchanger system comprises a further oxidation site for oxidising a further constituent of the flow of exhaust gases. Such a further oxidation site may be used to produce heat and thus to control the temperature within the heat exchanger system such that a relevant temperature interval may be achieved which in turn improves the control of the desired NO:NO2 ratio interval.
According to an embodiment the position of the downstream end of the NO oxidation site is adapted to the size and position of the further oxidation site. Hereby further control of the temperature interval and thus of the NO:NO2 ratio interval may be achieved.
According to an embodiment the NO oxidation site is positioned downstream of the further oxidation site. This is particularly relevant when the further oxidation site is known to consume any one or both of the nitrogen monoxide and the nitrogen dioxide. In such a case the NO:NO2 ratio interval may be made to deviate from the NO:NO2 ratio interval and the advantage of the present disclosure is reduced accordingly. A known consumer of these constituents is a methane oxidation site.
According to an embodiment the heat exchanger comprises a first guiding passage, a second guiding passage and a flow reversing region there between, whereby the first guiding passage is arranged to guide the flow of exhaust gases along a first flow direction towards the flow reversing region, and the second guiding passage is arranged to guide the flow of exhaust gases from the flow reversing region in a second flow direction, such that heat may be exchanged between the flow of exhaust gases in the second guiding passage and the flow of exhaust gases in the first guiding means. Hereby also a compact heat exchanger system is achieved.
According to an embodiment the heat exchanger system is a counter-flow heat exchanger, whereby the second guiding passage is arranged to guide the flow of exhaust gases in the second flow direction which is generally opposite to the first flow direction of the first guiding passage.
According to an embodiment the NO oxidation site is positioned within the second guiding passage. In this location the temperature is generally the highest within the heat exchanger system and the potential for raising the temperature to desired levels to achieve the desired NO:NO2 ratio interval is the greatest.
According to an embodiment the further oxidation site is positioned within the first guiding passage. Thereby the temperature will rise over the further oxidation site which heat will be used to control the temperature at the NO oxidation she.
According to an embodiment the NO oxidation site has an upstream end opposite to the downstream end, which upstream end is positioned immediately adjacent the flow reversing region. Hereby the largest possible surface area is attained for the NO oxidation site. The surface area, i.e. an active surface area, of the NO oxidation site may consequently be adapted such that e.g. a dwell time of the flow of exhaust gases in the proximity of, or over, the NO oxidation site is maximised to achieve that the chemical reaction taking place at the NO oxidation site is given the particular conditions to reach the intended equilibrium state indicated by the earlier mentioned equilibrium curve at the downstream end of the NO oxidation site.
According to an embodiment the further oxidation site is positioned immediately adjacent the flow reversing region. This will improve the temperature for the oxidising reaction for the further oxidation site.
According to an embodiment the further oxidation site is a hydrocarbon oxidation site for oxidising hydrocarbon (HC) to mainly carbon dioxide and water (H2O), more preferably for oxidising methane (CH4). Heat produced in particular by the exothermic methane oxidation may be exchanged within the heat exchanger system so that the required operating temperature for the methane oxidation at the further oxidation site is ensured as well as for the nitrogen oxidation at the NO oxidation site.
According to an embodiment the desired NO to NO2 (NO:NO2) ratio interval is 30:70-70:30 more preferably 40:60-60:40 and most preferably 45:55-55:45.
According to an embodiment the predetermined temperature interval is 350-420° C. more preferably 380-410° C. and most preferably 390-400° C.
According to an embodiment the NO oxidation site comprises an NO oxidation catalyst.
According to an embodiment the further oxidation site comprises a catalyst for oxidising the further constituent.
According to an embodiment an exhaust gas inlet is arranged to the first guiding passage, and an exhaust gas outlet is arranged to the second guiding passage.
According to an embodiment the downstream end of the NO oxidation site is positioned within a middle third part between the flow reversing region and a downstream end of the second guiding passage, preferably within a middle fourth part thereof, most preferably within a middle fifth part thereof. The inventors have realised that by coating approximately half of the second guiding means, corresponding to a downstream end of the NO oxidation site positioned within the middle fifth of the second guiding passage a desired NO:NO2 ratio of approximately 50:50 is achieved, which is an often desired ratio.
According to an embodiment a surface area of the second guiding passage is coated by the NO oxidizing catalyst to at least 33%, more preferably to at least 38%, and most preferably to at least 40%. This will achieve a desired NO:NO2 ratio interval.
According to an embodiment the heat exchanger system further comprises a third guiding passage for guiding at least part of the flow of exhaust gases from the exhaust gas inlet to a second flow reversing region, and a fourth guiding passage for guiding the flow of exhaust gases from the second flow reversing region to the exhaust gas outlet. Hereby the flow of exhaust gases may be divided into at least two sub-flows in order to treat the sub-flows differently to better control the desired result. In other words, the heat exchanger system may be split into two heat exchangers or heat exchanging regions, which may be arranged in physically separate devices. Alternatively, it is also possible to provide both heat exchangers in one system, e.g. in a heat exchanger system comprising at least four passages, wherein a first part of the passages, preferably approximately half of the passages, belong to the fast heat exchanger region and the other part belong to the second heat exchanger region. It should also be noted that the first flow reversing region and the second flow reversing region may be the same flow reversing region.
According to an embodiment the third guiding passage comprises a further oxidation site for oxidising a further constituent of the flow of exhaust gases. This may be either a similar or a different oxidation site with regard to the further oxidation site already mentioned earlier.
According to an embodiment the further oxidation site of the third guiding passage is a hydrocarbon oxidation site for oxidising hydrocarbon (HC) to mainly carbon dioxide (CO2) and water (H2O), more preferably for oxidising methane (CH4).
It may be preferred to coat the first and third guiding passage with the methane oxidizing catalyst material, but to coat only the second guiding passage with the nitrogen oxidizing catalyst. Thereby, the NO oxidation site may according to one embodiment cover approximately the complete surface area of the second guiding passage for oxidizing almost all nitrogen monoxide present in the flow of exhaust gases streaming through the second guiding passage to nitrogen dioxide. Consequently, the fourth guiding, passage guides unconverted nitrogen monoxide and the second guiding passage guides oxidized nitrogen dioxide to the common outlet, where both flows of exhaust gases are mixed so that the exhaust gas exiting the heat exchanger system lies within the desired NO:NO2 ratio interval, and more preferably in the vicinity of a 50:50 ratio.
According to an embodiment the further oxidation site (xx) of the third guiding passage comprises a catalyst for oxidising the further constituent.
According to an embodiment the fourth guiding passage comprises a selective catalytic reduction site for selective reduction of NO and NO2 in the flow of exhaust gases to mainly nitrogen (N2). A selective catalytic reduction (SCR) site reduces the total NOx levels within the flow of exhaust gases.
Since methane oxidation produces enough thermal energy for operating an SCR unit it is possible to reduce the NOx amount of the exhaust gas already at an early stage. Since a reduction agent, preferably urea, may be beneficial for the selective catalytic reduction reaction, it is preferred to provide the reduction agent to the exhaust gas upstream of the selective catalytic reduction coating. Even if the reduction agent injection may be arranged upstream of the heat exchanger system itself, it is more preferred to arrange the reduction agent injection device in the second flow reversing region since the temperature downstream of the methane oxidation catalyst, i.e. at the flow reversing region, is high enough to ensure that solid urea deposits in the system may be avoided. Additionally in the case of urea as reduction agent, the high temperatures allow for a conversion of urea to ammonia, which in turn increases the efficiency of the selective catalytic reduction.
According to an embodiment the fourth guiding passage comprises a selective catalytic reduction catalyst.
According to an embodiment the heat exchanger system comprises a first heat exchanger device incorporating the first and second guiding passages and the first flow reversing region, and a second heat exchanger device incorporating the third and fourth guiding passages and the second exhaust gas flow reversing region. If the first heat exchanger device and the second heat exchanger device are made as separate arrangements, they have the advantage that each device is less space demanding and that they may be arranged at different locations in a vehicle. On the other hand if both heat exchanger devices are incorporated into a single device, a compact heat exchanger system may be provided which is easily arranged in an exhaust gas aftertreatment system.
According to an embodiment the flow of exhaust gases through the exhaust gas inlet is adapted to be distributed between the first and third guiding passages, wherein the flow of exhaust gases preferably is adapted to be distributed generally evenly between the first and third guiding passages. This way a controllable NO:NO2 ratio and/or NO:NO2 ratio interval may be achieved.
According to an embodiment the heat exchanger system comprises an exhaust gas distribution device, particularly a valve, for controlling the amount of exhaust gas through the first and third guiding passages, respectively.
According to an embodiment the exhaust gas distribution device is adapted to be controlled in accordance with a sensed NO amount and/or NO2 amount and/or the NO:NO2 ratio in the flow of exhaust gases.
According to an embodiment the first and/or second flow reversing region is equipped with at least one urea injection device for injecting urea into the ex-haunt gas. Injection of urea into the exhaust gas improves the performance of the SCR site in an otherwise known manner.
According to an embodiment the first and/or second flow reversing region is equipped with at least one heater. A heater tray be used to control the temperature within the heat exchanger system, particularly for providing enough heat at a cold start or during low load application.
According to a second aspect of the disclosure an exhaust gas aftertreat-ment system for controlling exhaust gas emissions of an internal combustion engine, particularly at least the emissions of hydrocarbons and/or nitrogen oxides, is disclosed, comprising a heat exchanger system according to the first aspect of the disclosure. The exhaust gas aftertreatment system will gain similar or corresponding advantages as are disclosed in relation to the first aspect of the present disclosure above.
According to an embodiment a NOx sensor is arranged downstream of the heat exchanger system for sensing an NO amount and/or NO2 amount and/or an NO:NO2 ratio in the flow of exhaust gases leaving the heat exchanger system. A more detailed control of the exhaust gas aftertreatment system may thus be achieved.
According to an embodiment the exhaust gas aftertreatment system comprises a selective catalytic reduction unit and optionally a particulate filter, and wherein the heat exchanger system is arranged upstream of the selective catalytic reduction unit, preferably also upstream of the optional particle filter. The proper functionality of a selective catalytic reduction unit is dependent on the NO:NO2 ratio within the exhaust gases passing through it. Hence a combination of this kind improves exhaust emission control.
Moreover, in case of a heat exchanging system comprising a selective catalytic reduction site and a reduction agent injection, unused reduction agent from this heat exchanging system, such as urea or ammonia, may be transported to the selective catalytic reduction unit arranged downstream of the heat exchanger system. Alternatively or additionally to the reduction agent injection in the second flow reversing region, it is also possible to provide a reduction agent injection upstream of the selective catalytic reduction unit.
According to an embodiment the exhaust gas aftertreatment system comprises an oxidation catalyst, wherein the heat exchanger system is arranged downstream of the oxidation catalyst or in a bypass passage bypassing the oxidation catalyst. The oxidation catalyst not only controls the contents of the exhaust gases, but also the temperature of the exhaust gases.
According to a third aspect of the present disclosure a method for controlling exhaust gas emissions of an internal combustion engine is disclosed which comprises the step of using a heat exchanger system according to the first aspect and/or an exhaust gas aftertreatment system according to the second aspect. The method is given similar or corresponding advantages as are presented for the first and second aspects of the present disclosure.
According to a fourth aspect of the present disclosure a vehicle is disclosed which comprises a heat exchanger system according to the first aspect or an exhaust gas aftertreatment system according to the second aspect. The vehicle is given similar or corresponding advantages as are presented for the first, second and third aspects of the present disclosure.
According to an embodiment an engine of the vehicle is adapted to operate using compressed natural gas (CNG) or liquid natural gas (LNG). CNG and LNG both comprise methane, which according to the above gives an exothermic reaction when oxidised. Both also have other advantages, such as existing distribution networks in certain areas and other characteristics which have made them a focus for research into alternative fuels.
Further advantages and preferred embodiments are defined in the appending claims, the description and the figures.